Present day wireless communication applications demand high speed data transmissions. This is made possible by multilevel modulation schemes like Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). The non-linear effects in RF amplifiers such as gain compression cause power efficiency problems especially in multilevel modulation scheme causing a trade off. There are methods to compensate for these non-linear effects.
In one compensation method, the input power to the amplifier is decreased, but this leads to power efficiency losses. Compound semiconductors like GaAs can also be used in the RF amplifiers, but that will increase the cost of the amplifier, making it unsuitable for low cost and small scale integrated circuit implementations.
One type of digital modulation, PSK, uses distinct phases and each of these phases represents a unique symbol pattern of bits. There are in-phase and quadrature components of the modulated signal. Generally, after modulation, the symbols will be complex (x+iy). If the signal values fall beyond a particular threshold in the scatterplot then it will be detected as another symbol thus causing errors.
QAM is both a digital scheme as well as an analog scheme. It conveys two analog signals or digital bit streams using amplitude shift keying and these are usually 90 degrees out of phase—and so the name quadrature. The gain in RF amplifiers is not linear. At higher input levels the gain tends to saturate. This effect is called gain compression and the parameter to signify gain compression is called the 1 dB compression point. The 1 dB compression point may be defined as the input power at which the output is 1 dB less than the expected value. The system equation of a linear power amplifier may be given asy(t)=a1*x(t)  (1)
The system equation of the RF amplifiers can be generalized due to non-linearities as:y(t)=a1*x(t)+a2*x2(t)+a3*x3(t)  (2)
Gain compression causes errors in both amplitude and phase (because the symbols generally of the form x+iy and x and y are separately amplified). To have a high speed data rate, multilevel modulation schemes may be used. The previous observations show that effects of non-linearity in RF amplifiers cause severe performance losses for these modulation schemes. There are heretofore unaddressed needs with previous solutions.